When you turn on the heater blower,  you will probably draw about 10 amps at
max position.   This means you will be consuming about 12 x 10 = 120 watts of
electrical power.
1 HP is 746 watts, depending on the unit system, so your fan is consuming
about 1/6 of a HP which even for the Vanagons engine is not that much.
The heater core is probably capable of rejecting about 25% of the engines
heat so by using the heater fan you can easily make the difference of
overheating an engine or not overheating it.    A good chunk of the energy
from gasoline is turned into heat with a combustion engine.
Very roughly speaking, a 100 HP engine can produce more than 60 HP in heat
loss which your radiator system has to reject.    At that rate your heater
core can reject 15 HP.
These are really really round numbers so dont be tempted to take me to task
over efficiencies etc.    I am attempting to show how effective using the
heater fan can be to cool an engine.
Not sure about the Vanagon set-up but most vehicles have the heater plumbed
into the water system upstream of the thermostat so if the thermostat is
defective and wont open, using the heater at full blast can allow you to
gently drive somewhere for help.
Hope this is clear,
Steve Cotsford